An air turbine starter (ATS) is a well-known device utilized to initiate spool rotation of an aircraft's gas turbine engine during main engine start (MES). A generalized ATS includes a housing assembly having a flow passage therethrough, a turbine wheel rotatably mounted in the housing assembly, an ATS output shaft, and a gear train mechanically coupled between the turbine wheel and the output shaft. Notably, the ATS housing assembly may comprise multiple individual housings or modules fixedly joined together. These housing may include, for example, a turbine module housing containing the turbine wheel. The ATS output shaft is mechanically coupled to a spool (e.g., the high pressure spool) of a gas turbine engine through an accessory gearbox mounted to the engine's exterior. During MES, pressurized air supplied to an inlet flows through the ATS flow passage and causes the turbine wheel to rotate from rest to starter cutout at a relatively high rate of speed (e.g., 70,000 to 120,000 revolutions per minute). The rotating turbine wheel drives the engine spool through the gear train, ATS output shaft, and accessory gearbox. As the engine spool rotates, so too does the turbine (or turbines) mounted to the engine spool. When the rotational rate of the turbine is sufficient to maintain a minimum airflow through the gas turbine engine, the gas turbine engine is started and the ATS is deactivated. During a typical MES procedure, the ATS may be active for a total of approximately 30 to 60 seconds. After MES, the ATS generally remains inactive for the remainder of the flight.
In addition to producing thrust, an aircraft's gas turbine engine may also be relied upon to provide electrical power. In particular, an electrical generator may be mounted to the accessory gearbox and driven by the gas turbine engine during normal operation. The electrical output produced by generator may then be routed to various electrical systems aboard the aircraft. When a gas turbine engine is utilized to drive an electrical generator in this manner, the electrical output of the generator is generally dictated by the core speed of the gas turbine engine or, more specifically, the rotational rate of the spool to which the generator is mechanically coupled. When the gas turbine engine produces relatively little thrust, such as during ground idle or descent, the speed of the gas turbine engine may be relatively low. In such instances, the electrical output of the generator is reduced. Although the speed of the gas turbine engine may be increased to augment the electrical output of the generator, this typically results in the production of a high forward thrust. The forward thrust may be reduced by dumping a portion of the air discharged from the gas turbine engine's turbine or turbines overboard; however, this practice still results in a net energy loss and is consequently inefficient.
Considering the above, it should be appreciated that it would be desirable to provide a starter/drive turbine system capable of providing a relatively efficient energy distribution between the low propulsion requirements of a gas turbine engine and the electrical load placed on an electrical generator. It would also be desirable if such a system employed a bi-modal turbine assembly capable of starting the gas turbine engine during MES and, thus, could replace a conventional ATS to reduce overall cost, part count, and weight. Other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description and the appended claims, taken in conjunction with the accompanying drawings and this Background.